Imaging device comprising an image-intensifying tube

ABSTRACT

The luminescent screen collecting the photo-electrons of an imaging device is provided with a cover layer that decelerates the electrons so that luminescence occurs only if the photoelectrons have been accelerated to at least a quarter and preferably at least one half of the value of the nominal operating voltage thereby increasing the control range of the permissible intensity variations within the acceptable image definition limits of the device.

United States Patent 1191 Ligtenberg Nov. 20, 197 3 IMAGING DEVICE COMPRISING AN 3,350,594 10/ 1967 Davis et a1 250/213 VT IMAGEJNTENSIFYING TUBE 3,232,781 2/1966 McGee 313/94 X 2,909,703 10/ 1959 Williams 250/213 VT Martinus Adrianus Cornelis Ligtenberg, Emmasingel, Eindhoven, Netherlands U.S. Philips Corporation, New York, N.Y.

Filed: Apr. 10, 1972 Appl. No.: 242,693

Inventor:

Assignee:

Foreign Application Priority Data Apr. 16, 1971 Netherlands 7105196 us. (:1 250 213 VT, 313/94, 313/108 R Int Cl. n01;- l/62, l-lOlj 31/50, 1101 j 39/00 Field of Search 250/213 VT; 313/94,

References Cited UNITED STATES PATENTS 9/1950 Mason et a1. 313/108 R Primary Examiner-James W. Lawrence Assistant ExaminerT. N. Grigsby Attorney-Frank R. Trifari 22 Claims, 2 Drawing Figures Fig. 2

IMAGING DEVICE COMPRISING AN IMAGE-INTENSIFYING TUBE The invention relates to an imaging device provided with an image-intensifying tube, comprising a photocathode and a luminescent layer for collecting electrons emitted by the photocathode, the side of the luminescent layer facing the photocathode being provided with a light-impervious, electrically conducting cover layer, said device comprising means for applying an adjustable acceleration voltage between the photocathode and the cover layer, and furthermore relates to an image-intensifying tube for use in a device of this kind.

In imaging devices of the kind set forth, the adjustability of the voltage to be applied to the tube is utilized for adjusting the intensity of the image produced by the luminescent screen. This adjustability is utilized in particular when displaying X-ray images, and the imageintensifying tube is a tube for intensifying X-ray images and hence is provided with a luminescent entrance screen which converts the X-rays into light and which is optically coupled to the photocathode. The control of the intensity of the image produced by the luminescent layer collecting the electrons is important if adaptation of the eye is involved, or when a television camera tube is coupled to the image-intensifying tube and control of the intensity of the image radiation collected by the image intensifier is not desirable or not very well possible, which is frequently the case when working with X-rays. In the case of intensity control by adjustment of the operating voltage of the tube, this operating voltage can practically not be decreased further than approximately half the value of the nominal operating voltage of the tube since the electron-optical system then ceases to produce a sufficiently defined image. The nominal operating voltage of the tube is to be understood to mean the accelerating voltage between photocathode and cover layer which ensures correct imaging and to which the construction of the tube (configuration of the electrodes, including the photocathode and the luminescent layer with cover layer collecting the electrons) is adapted.

In the known imaging devices of the kind set forth an intensity ratio of the luminescent image produced by the electrons of approximately 1 to one-third can be realized by controlling the voltage from the nominal operating voltage to approximately half the value thereof, which is often experienced to be too small.

The invention has for its object to provide a measure by which the control range of the said intensity can be substantially increased, said measure being based on the recognition of the fact that this can be achieved by constructing the cover layer such that with an everincreasing operating voltage of the tube, an everincreasing portion of the electrons impinging upon the cover layer lose their energy in this cover layer.

To this end, the imaging device according to the invention is characterized in that the nature of the material and the thickness of the cover layer are chosen to be such that at an operating voltage of the tube which is less than one quarter of the value of the nominal operating voltage of the tube the cover layer allows substantially no further electrons to pass which cause the luminescent layer to luminesce.

In an advantageous embodiment of the imaging device according to the invention, the cover layer is constructed such that luminescence of the luminescent layer occurs only at an operating voltage which amounts to 50 to percent of the nominal operating voltage. An image intensifying tube for use in the imaging device according to the invention, said tube having an entrance screen with a photocathode and a luminescent screen collecting electrons originating from the photocathode, said luminescent screen comprising a luminescent layer which is covered on the side of the photocathode by an electrically conducting, lightimpervious cover layer, is characterized in that the na ture of the material and the thickness of the cover layer are chosen to be such that at an operating voltage of the tube which is less than a quarter of the nominal operating voltage of the tube, the cover layer allows substantially no electrons to pass which cause the luminescent layer to luminesce. In an image intensifying tube of this kind the cover layer is preferably constructed such that luminescence of the luminescent layer occurs only at an operating voltage which amounts to 50 to 80 percent of the nominal operating voltage of the tube.

The invention will be described in detail hereinafter with reference to some embodiments shown in the accompanying drawing. Therein:

FIG. 1 is a schematic view of an imaging device having an X-rayimage intensifier and an adjustable operating voltage, while FIG. 2 is a graph showing the relationship which is found between the operating voltage on the tube and the luminescence of the luminescent layer in the case of a commonly used X-ray image intensifier and in the case of different X-ray image intensifiers to be used in the imaging device according to the invention.

In the imaging device which is schematically shown in FIG. 1, X-rays emitted by an X-ray tube 1 are col lected, after having passed an object 2 to be examined, by the entrance screen 4 of an X-ray image-intensifying tube 3. This tube 3 has a glass envelope 5 and comprises a curved front face 6, the inner side of which ac commodates the entrance screen 4. This entrance screen 4 is composed of a luminescent layer 7 which is provided directly on the glass of the front face 6, a thin transparent separating layer 8, and a photocathode layer 9 which is sensitive to the luminescent light of the layer 7. At the other end of the envelope 5, the tube is provided with an exit window 10 on which is provided an exit screen 11 consisting of a luminescent layer 12 which is covered on the side facing the entrance screen 4 by an electrically conducting, light-impervious cover layer 13. The exit screen is surrounded by and electrically connected to a funnel-like anode 14. Furthermore, a rotating intermediate electrode 15 is provided on the inner wall of the envelope 5.

For the electrical supply of the X-ray imageintensifying tube 3 a voltage source 20 is provided, which is bridged by a resistor 21 which is constructed as a potentiometer. The photocathode 9, the intermediate electrode 15, and the anode 14 together with the exit screen 11 are connected to this potentiometer, the photocathode being connected to an adjustable contact 22, the anode 14 and the exit screen 11 together being connected to an adjustable contact 23. The intermediate electrode 15 is connected to a fixed point of the po tentiometer 21, so that this intermediate electrode 15 carries a positive potential of 200 to 250 volts with respect to the photocathode 9. The positive voltage (tube voltage) V, carried by the anode 14 together with the exit screen 11 with respect to the photocathode 9, is

adjustable from the voltage of the voltage source V equal to or slightly higher than the nominal operating voltage V of the image-intensifying tube 3 to approximately one quarter of the value thereof. The nominal operating voltage V of an image-intensifying tube is to be understood to mean the voltage to be applied between the photocathode and the luminescent layer collecting the photo-electrons for obtaining the most favourable electron-optical image of the photocathode on the luminescent layer at the given tube configuration.

The intensity of the image produced by the luminescent layer 12 can be varied by adjusting the tube voltage V by means of the contacts 22 and 23, the former contact serving to enable adaptation of the voltage difference between the photocathode 9 and the intermediate electrode to the changing tube voltage.

Thus far the device shown in FIG. 1 does not differ from known devices.

In these known devices the cover layer 13 is formed on the luminescent layer 12 by a thin layer of vapourdeposited aluminum (the so-termed metal backing), having a thickness of 1,000 to a maximum of 3,000 A, the common value being approximately 1,250 A.

A first object of such a layer also commonly used in television display tubes and other cathode-ray tubes having a cathode luminescent layer is to reflect the light transmitted to the cover layer by the luminescent layer, a second object being the conducting away of the electrons impinging on the exit screen, while a layer of this kind can also serve to prevent interaction between the material of the photocathode, for example, cae sium, and that of the luminescent layer. An aluminum layer having the above-mentioned thickness of 3,000 A is permeable to electrons having a velocity of 3 to 4 kV. The penetration depth S of electrons into a material generally satisfies the formula S Cv lp, in which C is a constant, p is the specific gravity of the layer material, and V is the voltage by which the electrons have been accelerated when they reach the layer.

In an imaging device according to the invention and constructed as shown in FIG. 1, the cover layer 13 in the imageintensifying tube is considerably thicker than usual, and it may also consist of different sublayers of different material. The decisive fact is that this cover layer is less permeable to electrons than the commonly used cover layer, so that electrons originating from the photocathode 9 can penetrate the cover layer and cause the luminescent layer 12 to luminesce only if the tube voltage is higher than the above-mentioned acceleration voltage of 3 to 4 kV. According to the invention, the cover layer 13 is to be constructed such that photoelectrons cause luminescence only at a tube voltage V which amounts to one quarter of the value of the nominal operating voltage V this should preferably occur only at a tube voltage amounting from 50 to 80 percent of the nominal operating voltage V, of the image-intensifying tube 3.

The effect of the measure according to the invention can be illustrated by means of FIG. 2, in which the variation in the luminance I of the exit screen is plotted as a function of the tube voltage V, for cover layers of different photoelectron-permeability, the luminance at the nominal operating voltage V being each time assumed to be equal to l. A nominal operating voltage of 22 to 25 kV is commonly used for image-intensifying tubes; FIG. 2 relates to an X-ray image intensifier where V, 25 kV.

Curve a in FIG. 2 relates to an X-ray image intensifier having the usual aluminum cover layer of a thickness of 1,000 to 2,000 A, curves 1;, c and d relating to X-ray image intensifiers in the device according to the invention. Curve b applied to an X-ray image intensifier having a cover layer consisting of an aluminum layer of a thickness of approximately 10,000 A which is only permeable to electrons which have been accelerated beyond approximately 8 kV. Curves c and d apply to image intensifiers whose cover layer is permeable only to electrons accelerated beyond 12.5 kV, i.e. half the nominal operating voltage V, or more than 16 kV, respectively.

When the device as described with reference to FIG. 1 is used, it is impracticable to reduce the tube voltage V to less than half the nominal operating voltage V, and still obtain an image of reasonable quality. This is inherent in the electron-optical properties of the image intensifier. Interpreted in FIG. 2, this means that the luminance of the luminescent layer 12 of the image intensifier can in practice be controlled by varying the adjustment of the tube voltage only in accordance with the portion of the curves a, b, c and d which is situated to the right of the dotted line V k V If the control range of the luminance is understood to mean the ratio between the luminance at the nominal operating voltage V,, and the minimum luminance achievable without decreasing the tube voltage to less than half the nominal operating voltage, FIG. 2 clearly illustrates that such a control range is approximately 3 in the known image intensifier (curve a), this range being approximately 10 in the image intensifier to be used in the device according to the invention and provided with an aluminum cover layer having a thickness of 10,000 AE, while for image intensifiers which are subject to the curves c and d the control range is substantially larger yet; in practice it can even be as high as 100. The measure according to the invention, consequently, provides a substantially larger control possibility for adaptation of the luminance of the exit screen to, for example, the eye or a television camera tube than is possible with the known devices.

Because the deposition, i.e., vapour-deposition, of an aluminum cover layer having a thickness exceeding approximtely 10,000 A is difficult, it is desirable to use a different method of forming cover layers which are only penetrated by electrons which have been accelerated beyond 8 kV. For example, other metals such as titanium, silver, lead and especially gold can be used, it often being advantageous to use a thin layer of aluminum on the side of the photocathode and also on the side of the luminescent layer so as to avoid contamination. The cover layer is then composed of a number of sublayers. An example thereof is a cover layer consisting of three sublayers, i.e. a first layer of aluminum having a thickness of about 1,000 A which is provided directly on the luminescent layer, a gold or lead layer having a thickness of 3,000 to 6,000 A which is provided on this aluminum layer, and on this lead or gold layer a second aluminum layer having a thickness of, for example, approximately 600 A. Another example is a titanium layer having a thickness of approximately 1,200 A which is provided directly on the luminescent layer, an aluminum layer of approximately 1,000 A being provided on the side thereof which faces the photocathode 9. A guide for determining the required thickness of a layer or sublayer is the fact that thicknesses of different materials which are equivalent for' electron absorption relate inversely to the specific gravities of those materials. Other possibilities of realizing a cover layer in accordance with the measure of the invention can be found in the application of metal oxides such as titanium monoxide (TiO), specific gravity 4.93 consequently, almost twice as high as that of aluminum and titanium dioxide, specific gravity 4.l7 or 3.84, depending on the modification. Another possibility yet can be found in the form of a sublayer of luminescent material, for example, the same as used in the luminescent layer 12, however, without activator so that this material absorbs electrons without radiation of light. A suitable material for this purpose is zinc cadmium sulphide (Zn CdS) without activator, the specific gravity (approximately 4.5) of which is slightly more than one and a half times that of aluminum. An example in this respect is a cover layer 13 consisting of two sublayers: a first layer directly adjoining the luminescent layer 12 and composed of a sublayer of zinc cadmium sulphide and having a thickness of approximately 15,000 A which is obtained, for example, by settling, and vapour-deposited thereon an aluminum layer having a thickness of 500 to 1,000 A.

I claim:

1. An image-intensifying tube comprising a photocathode, a luminescent screen spaced apart from said photocathode for collecting electrons originating from the photocathode, an electrically conducting, lightimpervious cover layer on the side of said luminescent layer facing the photocathode, the material and the thickness of the cover layer being such that the cover layer substantially prevents electrons from passing through that cause the luminescent screen to luminesce at voltages less than one quarter of the value of the nominal operating voltage of the tube.

2. An image-intensifying tube as claimed in claim 1, wherein the cover layer is such that luminescence of the luminescent layer initially occurs at voltages from 50 to 80 percent of the nominal operating voltage of the tube.

3. An image-intensifying tube as claimed in claim 1, wherein the cover layer is formed by a layer of aluminum having a thickness of approximately 10,000A.

4. An image-intensifying tube as claimed in claim 1, wherein the cover layer is formed by a plurality of mutually adjoining sublayers, at least one of said sublayers being made of metal.

5. An image-intensifying tube as claimed in claim 4, wherein said sublayer of the cover layer adjoining the luminescent layer essentially consists of a luminescent material which produces minimum light emission when electrons originating from the photo-cathode penetrate said material.

6. An image-intensifying tube as claimed in claim 4, wherein the cover layer comprises two layers of aluminum and a layer of gold provided between said aluminum layers.

7. An image-intensifying tube as claimed in claim 6, wherein the cover layer comprises an aluminum layer adjoining the luminescent layer, said luminescent layer having a thickness of approximately 1,000A, said gold layer having a thickness of 3,000 to 6,000A, and said second aluminum layer having a thickness of 500 to 1,000A on the side facing the photocathode.

8. An image-intensifying tube as claimed in claim 4, wherein the cover layer comprises a sublayer of a metal oxide.

9. An image-intensifying tube as claimed in claim 8, wherein the metal oxide sublayer essentially consists of titanium oxide.

10. An image-intensifying tube as claimed in claim 4, wherein the cover layer comprises at least one aluminum layer and a sublayer of a second metal.

11. An imageintensifying tube as claimed in claim 10, wherein the sublayer of said second metal is selected from the group consisting of titanium, silver and lead.

12. An imaging system for an image-intensifying tube, comprising a photocathode, a luminescent layer positioned for collecting electrons emitted by said photocathode, an electrically conducting, light impervious cover layer on the side of said luminescent layer facing the photocathode, and means for applying an adjustable acceleration voltage between said photocathode and said cover layer, said tube having a nominal operating voltage V, and undergoing decelerates changes in the image produced thereby upon reduction of the operating voltage thereof to a value less than one-fourth of said nominal voltage value, the dimensions and characteristics of said cover layer beingchosen so as to substantially prevent electrons originating in said photocathode from causing said luminescent screen to luminesce at acceleration voltages between said photocathode and said cover layer less than one quarter of the nominal operating voltage of said tube.

13. An imaging system as claimed in claim 12, wherein the cover layer is such that luminescence of the luminescent screen initially occurs at voltages of from 50 to percent of the nominal operating voltage of said tube.

14. An imaging system as claimed in claim 12, wherein the cover layer is formed by a thin layer of aluminum having a thickness of approximately 10,000A.

15. An imaging system as claimed in claim 12, wherein the cover layer is formed from a plurality of mutually adjoining sublayers, at least one of said sublayers being made of metal.

16. An imaging system as claimed in claim 15, wherein said sublayer adjoining the luminescent layer essentially consists of luminescent material having substantially minimum light emission when electrons originating from the photocathode penetrate said material.

17. An imaging system as claimed in claim 15, wherein the cover layer comprises two layers of aluminum and one layer of gold provided between said aluminum layers.

18. An imaging system as claimed in claim 17, wherein the cover layer comprises an aluminum layer adjoining the luminescent layer, said luminescent layer having a thickness of approximately 1,000A, said gold layer having a thickness of 3,000 to 6,000A and said second aluminum layer having a thickness of 500 to 1,000A on the side facing the photocathode.

19. An imaging system as claimed in claim 15, wherein the cover layer comprises a sublayer of a metal oxide.

20. An imaging system as claimed in claim 19 wherein the metal oxide sublayer essentially consists of titanium oxide.

21. An imaging system as claimed in claim 15, wherein the cover layer comprises at least one aluminum layer and a sublayer of a second metal.

22. An imaging system as claimed in claim 21, wherein the sublayer of said second metal is selected from the group consisting of titanium, silver and lead. 

1. An image-intensifying tube comprising a photocathode, a luminescent screen spaced apart from said photocathode for collecting electrons originating from the photocathode, an electrically conducting, light-impervious cover layer on the side of said luminescent layer facing the photocathode, the material and the thickness of the cover layer being such that the cover layer substantially prevents electrons from passing through that cause the luminescent screen to luminesce at voltages less than one quarter of the value of the nominal operating voltage of the tube.
 2. An image-intensifying tube as claimed in claim 1, wherein the cover layer is such that luminescence of the luminescent layer initially occurs at voltages from 50 to 80 percent of the nominal operating voltage of the tube.
 3. An image-intensifying tube as claimed in claim 1, wherein the cover layer is formed by a layer of aluminum having a thickness of approximately 10,000A.
 4. An image-intensifying tube as claimed in claim 1, wherein the cover layer is formed by a plurality of mutually adjoining sublayers, at least one of said sublayers being made of metal.
 5. An image-intensifying tube as claimed in claim 4, wherein said sublayer of the cover layer adjoining the luminescent layer essentially consists of a luminescent material which produces minimum light emission when electrons originating from the photo-cathode penetrate said material.
 6. An image-intensifying tube as claimed in claim 4, wherein the cover layer comprises two layers of aluminum and a layer of gold provided between said aluminum layers.
 7. An image-intensifying tube as claimed in claim 6, wherein the cover layer comprises an aluminum layer adjoining the luminescent layer, said luminescent layer having a thickness of approximately 1,000A, said gold layer having a thickness of 3,000 to 6,000A, and said second aluminum layer having a thickness of 500 to 1, 000A on the side facing the photocathode.
 8. An image-intensifying tube as claimed in claim 4, wherein the cover layer comprises a sublayer of a metal oxide.
 9. An image-intensifying tube as claimed in claim 8, wherein the metal oxide sublayer essentially consists of titanium oxide.
 10. An image-intensifying tube as claimed in claim 4, wherein the cover layer comprises at least one aluminum layer and a sublayer of a second metal.
 11. An image-intensifying tube as claimed in claim 10, wherein the sublayer of said second metal is selected from the group consisting of titanium, silver and lead.
 12. An imaging system for an image-intensifying tube, comprising a photocathode, a luminescent layer positioned for collecting electrons emitted by said photocathode, an electrically conducting, light impervious cover layer on the side of said luminescent layer facing the photocathode, and means for applying an adjustable acceleration voltage between said photocathode and said cover layer, said tube havinG a nominal operating voltage Vn and undergoing decelerates changes in the image produced thereby upon reduction of the operating voltage thereof to a value less than one-fourth of said nominal voltage value, the dimensions and characteristics of said cover layer being chosen so as to substantially prevent electrons originating in said photocathode from causing said luminescent screen to luminesce at acceleration voltages between said photocathode and said cover layer less than one quarter of the nominal operating voltage of said tube.
 13. An imaging system as claimed in claim 12, wherein the cover layer is such that luminescence of the luminescent screen initially occurs at voltages of from 50 to 80 percent of the nominal operating voltage of said tube.
 14. An imaging system as claimed in claim 12, wherein the cover layer is formed by a thin layer of aluminum having a thickness of approximately 10,000A.
 15. An imaging system as claimed in claim 12, wherein the cover layer is formed from a plurality of mutually adjoining sublayers, at least one of said sublayers being made of metal.
 16. An imaging system as claimed in claim 15, wherein said sublayer adjoining the luminescent layer essentially consists of luminescent material having substantially minimum light emission when electrons originating from the photocathode penetrate said material.
 17. An imaging system as claimed in claim 15, wherein the cover layer comprises two layers of aluminum and one layer of gold provided between said aluminum layers.
 18. An imaging system as claimed in claim 17, wherein the cover layer comprises an aluminum layer adjoining the luminescent layer, said luminescent layer having a thickness of approximately 1,000A, said gold layer having a thickness of 3,000 to 6,000A and said second aluminum layer having a thickness of 500 to 1,000A on the side facing the photocathode.
 19. An imaging system as claimed in claim 15, wherein the cover layer comprises a sublayer of a metal oxide.
 20. An imaging system as claimed in claim 19 wherein the metal oxide sublayer essentially consists of titanium oxide.
 21. An imaging system as claimed in claim 15, wherein the cover layer comprises at least one aluminum layer and a sublayer of a second metal.
 22. An imaging system as claimed in claim 21, wherein the sublayer of said second metal is selected from the group consisting of titanium, silver and lead. 